Apelin, a recently isolated neuropeptide that is expressed in the supraoptic and the paraventricular nuclei, acts on specific receptors located on vasopressinergic neurons. The increased phasic pattern of these neurons facilitates sustained antidiuresis during dehydration or lactation. Here, we investigated whether apelin interacts with arginine vasopressin (AVP) to maintain body fluid homeostasis. We first characterized the predominant molecular forms of endogenous hypothalamic and plasma apelin as corresponding to apelin 13 and, to a lesser extent, to apelin 17. We then demonstrated that, in lactating rats, apelin was colocalized with AVP in supraoptic nucleus magnocellular neurons and given intracerebroventricularly inhibited the phasic electrical activity of AVP neurons. In lactating mice, intracerebroventricular administration of apelin 17 reduced plasma AVP levels and increased diuresis. Moreover, water deprivation, which increases systemic AVP release and causes depletion of hypothalamic AVP stores, decreased plasma apelin concentrations and induced hypothalamic accumulation of the peptide, indicating that AVP and apelin are conversely regulated to facilitate systemic AVP release and suppress diuresis. Opposite effects of AVP and apelin are likely to occur at the hypothalamic level through autocrine modulation of the phasic electrical activity of AVP neurons. Altogether, these data demonstrate that apelin acts as a potent diuretic neuropeptide counteracting AVP actions through inhibition of AVP neuron activity and AVP release. The coexistence of apelin and AVP in magnocellular neurons, their opposite biological effects, and regulation are likely to play a key role for maintaining body fluid homeostasis.A pelin is a bioactive peptide recently isolated from bovine stomach extracts (1). It was identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ (1, 2), reported to act as a coreceptor of CD4 for human and simian immunodeficiency viruses (3, 4). Apelin is a 36-aa peptide derived from a 77-aa precursor, preproapelin, for which cDNAs have been cloned from humans, cattle, rats, and mice (1, 5, 6). The apelin precursor has a fully conserved C-terminal sequence between Trp-55 to Phe-77, including the C-terminal 17 (Lys-Phe-Arg-ArgGln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe; K17F) and 13 (Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe; Q13F) amino acid sequences. These molecular species, and the pyroglutamyl form of Q13F (pE13F), exhibit the highest activities on extracellular acidification rate (1) and strongly inhibit forskolinstimulated cAMP production in Chinese hamster ovary (CHO) cells expressing the human (5,7,8) or the rat (9) apelin receptor. These peptides also are highly potent inducers of rat apelin receptor internalization (10,11).In situ hybridization and RT-PCR studies have shown that the apelin precursor and apelin receptor mRNAs are expressed in various rat brain structures (6,8,9,12,13). Apelin-immunoreactive (IR) neurons are particularly abundant in ...

Bromocriptine or other dopamine agonists are usually effective for the treatment of prolactin-secreting adenomas. Five to 18% of prolactinomas, however, do not respond to such therapy. We have shown previously that such resistance to bromocriptine correlates with reduced binding to the D₂ receptor subtype of dopamine, the major PRL inhibiting factor. In the present work, we demonstrated that reduced binding actually corresponds to decreased expression of the gene coding for the D₂ receptor in the pituitary from bromocriptine-resistant patients, as shown by 4-fold lower levels of the corresponding mRNAs compared to those coding for actin. The existence of two D₂ receptor isoforms, D₂S and D₂L generated by alternative splicing, has been described in several tissues, including the pituitary. Both are negatively coupled to adenylyl cyclase and inhibit prolactin secretion, but, in addition, the shortest one (D₂S) is more efficiently coupled to phospholipase C. Consequently, we also investigated whether expression of a particular D₂ receptor isoform was preferentially affected in resistant adenomas. The proportion of messengers corresponding to the short receptor isoform (D₂S) was lower in resistant compared to responsive adenomas: D₂S/D₂L = 0.74 ± 0.08 and 1.00 ± 0.07, respectively. In parallel, much lower levels of D₂ receptor mRNAs were found in growth hormone-secreting adenomas, with a D₂S/D₂L ratio comparable to those of both normal human pituitary and bromocriptine-sensitive prolactinomas (1.05 ± 0.11). Thus, resistance to bromocriptine therapy seems to involve defects in D₂ dopamine receptor expression and possibly in posttranscriptional splicing.

The forebrain and pituitary sites of synthesis of growth hormone secretagogue-receptor mRNA were identified in four adult lemurs (Microcebus murinus) by in situ hybridisation performed with a radiolabeled cRNA probe transcribed from human Growth Hormone Secretagogue-Receptor cDNA. The cRNA sense and antisense probes were hybridised to cryostat sections containing structures extending from the rostral hypothalamus to its caudal limit as defined by the mammillary bodies. The pituitary gland and areas adjacent to the hypothalamus were also analyzed. For comparative purposes, sections from five adult rats containing these structures were hybridised with the same probes. The results point to a widespread distribution of Growth Hormone Secretagogue-Receptor mRNA in the hypothalamus, hippocampal formation, and cerebellar cortex of both lemurs and rats. As in the rat, specific hybridisation was particularly dense in the arcuate nucleus. Significant species differences were observed in the periventricular nucleus, the ventromedial nucleus, the lateral hypothalamic area, and the pituitary gland. In contrast to the rat, the lemur exhibited marked labelling in the infundibular nucleus, the periventricular nucleus and the pars tuberalis of the pituitary gland, whereas no labeling was detectable in the ventromedial nucleus and the lateral hypothalamic area. These results are discussed in terms of difference between the control of growth hormone secretion, feeding behaviour and seasonal rhythmicity among murine species and primates.